1. Field of the Invention
This invention relates to an edge detecting device in an optical measuring instrument for measuring a dimension, a displacement value and the like of an object, and more particularly to an edge detecting device in an optical measuring instrument, wherein the object to be measured, which is not transparent, is directly irradiated by a scanning light, a transmitted light or a reflected light generated due to this irradiation, or a projected image of an object to be measured, generated due to the transmitted light or reflected light is received by a photo-electric element or elements to take out an electric signal or signals, and the measurement of a dimension, discrimination of a position, judgment of a configuration or the like of the object to be measured is performed in response to the aforesaid electric signal or signals.
2. Description of the Prior Art
The optical measuring instrument of the type described, such for example as a projector, has heretofore been of such an arrangement that an object to be measured on a mount is irradiated by a parallel light, a projected image of the object to be measured is made to focus on a screen in response to a transmitted light or a reflected light thereof, and a dimension, configuration or the like of the object to be measured is measured from the formed image. However, an edge of the image of the object to be measured, projected on the screen has a so-called bleading, in general. In consequence, it is difficult for the object to be measured to be accurately read in through a coincidence between the image formed on the screen and a hair line.
To obviate the above-described disadvantages, there has been proposed such a method that the edge of the formed image is moved relative to a photo-electric element, whereby a variation in value of an electric signal outputted from the photo-electric element due to a variation in a ratio between areas of a bright portion and a dark portion of the image projected on a light receiving surface of the photo-electric element is compared with a reference voltage, so that the edge of the projected image can be detected.
However, this method presents such disadvantages that the adverse influence by noises of lights of disturbance and the like is great and the measuring accuracy is lowered to a considerable extent by fluctuations in the signal obtained from the photo-electric element or the reference voltage.
Further, there is another method, wherein the photo-electric element is moved relative to the boundary (edge) of the image projected on the screen, an output signal at this time is subjected to the second order differentiation to obtain a wave form signal, whereby this wave form signal is compared with a reference voltage to thereby detect the edge. However, there are such disadvantages that positions of the edge detected are different depending upon the speeds of the relative movement between the photo-electric element and the projected image, and further, the measuring accuracy is lowered to a considerable extent by fluctuations in the reference voltage.
Further, there are provided two photo-electric elements, which are moved relative to the edge of the projected image, and a wave form signal is obtained from a plurality of output signals obtained by the aforesaid relative movement, whereby the wave form signal is compared with a reference voltage to thereby detect the edge. However, similarly to the above, there are presented such disadvantages that, due to a relative variation between the output signals and the reference voltage, fluctuations in level and the like, the measuring becomes highly unstable. Further, the scope of application for the intensity of illumination of the irradiating light is narrow, the made of measuring is restricted, and a sensor section or a circuit portion becomes complicated in construction.
Particularly, in the projector, the brightness of the image projected on the screen is changed due to the fatigue of a power source lamp for irradiation, the characteristics of lenses in a projection system and lights of disturbance, the brightness of the projected image is changed due to the switching of magnifications, and further, as a condition on the part of a measurer, since the brightness suitable to the operation is varied depending on the color of pupils of the measurer (varying with races) for example, the suitable brightness should be selected. As the result, the narrow scope of application for the intensity of illumination of the irradiating light as described above leads to lowered capacity of the projector.
Furthermore, according to the conventional method of detecting the edge, when the focus of the projected image is shifted, a wave form outputted by the photo-electric element becomes gentle, thus presenting the disadvantage of being incapable of accurately detecting the edge.
This disadvantage is not only of the projector but also common to the edge detections by the optical measuring instruments, wherein, in general, the transmitted light or reflected light is detected, whereby dimensions and the like of the object to be measured are directly or indirectly measured.
As against the above, as disclosed in Japanese Patent Kokai (Laid-open) No. 173408/83 for example, there is provided an edge detecting device in an optical measuring instrument wherein a transmitted light or a reflected light is detected to directly or indirectly measure a dimension of an object to be measured, comprising;
a sensor including four light receiving elements for producing at least two sets of phase shift signals in response to a bright portion or a dark portion which is generated at the time of relative movement with the object to be measured;
first and second calculating means for calculating differences between the phase shift signals of the respective sets;
a third calculating means for calculating a difference between signals outputted form the first and the second calculating means, and a fourth calculating means for calculating a sum between the signals outputted from the first and the second calculating means; and
sensing means for outputting a cross signal between a reference level signal and a signal outputted from the third calculating means, which is produced while a signal outputted from the fourth calculating means is on a predetermined level.
This edge detecting device can offer such advantages that the edge can be detected with a simplified construction and without the adverse influence due to the strength of the light irradiating the object to be measured, noises of the lights of disturbance and the like during measuring and fluctuations in the signals outputted from the photo-electric elements or in the reference voltage, the edge can be accurately detected even when there is a shift in focus of the projected image, and further, the edge of the object to be measured can be detected by directly processing an analogue signal from a photo-electric signal.
However, the edge detecting device disclosed in Japanese Patent Kokai (Laid-Open) No. 173408/83, having a sensor including four light receiving elements arranged in a shape of may not detect the edge of the projected image, in the case of a movement relative to an image on the screen in a projector for example, if the border line of the light receiving elements coincides with the direction of the movement. In consequence, such a disadvantage is presented that the direction of movement of the sensor relative to the projected image is restricted.